Pulse-width modulation (PWM) is a known type of modulation in which, typically, a signal cyclically alternates between two output states and signal values are encoded by the duration or width of pulses of different output states in each cycle. The signal value is typically encoded as the relative duration of a pulse of, say, a first state compared either to the duration of a pulse of a second state in the cycle or to the total cycle period. In some types of PWM the cycle period may be constant from one cycle to the next, i.e. there is a fixed cycle frequency.
PWM modulators may be used in a variety of applications. One particular application is as use of part of a signal path for converting a digital signal into an analogue signal, for example as part of an audio playback path for converting a digital audio signal into an analogue audio signal suitable for driving an audio transducer.
Conventionally, audio playback paths have been implemented using a suitable digital-to-analogue converter (DAC) implemented using analogue circuitry, e.g. a switched capacitor DAC or similar, to convert an input digital audio signal into an analogue signal. The analogue signal can be then be buffered or amplified as required, e.g. using any suitable amplifier such as class-AB type amplifier or an analogue-input Class-D type amplifier.
Increasingly however there is a trend to integrated circuits being implemented using smaller process node geometries. For such smaller process node geometries, analogue circuitry may present certain design challenges and may not scale well in terms of size and power requirements. Thus there is often a desire to implement as much circuitry as digital circuitry where possible.
Digital PWM generators can be used together with a Class-D output stage to provide a signal path that is predominantly digital and thus can be efficiently implemented on smaller process node geometries. The timing of edges of pulses is defined by a digital word and is quantised to edges of a fast clock. However digital PWM generation can require fast clock speeds to avoid distortion associated with the quantisation of the pulse position.